In recent years, a light emitting element using a light emitting organic compound has been actively researched and developed. A basic structure of this light emitting element is that a layer containing a light emitting organic compound (light emitting layer) is sandwiched between a pair of electrodes. By applying a voltage to this element, electrons and holes are separately transported from the pair of electrodes to the light emitting layer, and current flows. Then, recombination of these carriers (the electrons and holes) makes the light emitting organic compound to form an excited state and to emit light when the excited state returns to a ground state. Owing to such a mechanism, such a light emitting element is referred to as a current-excitation light emitting element.
Note that an excited state of an organic compound includes a singlet excited state and a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
A great advantage of such a light emitting element is that the light emitting element can be manufactured to be thin and lightweight, since the light emitting element is generally formed of an approximately submicron thin film. In addition, extremely high response speed is another advantage, since time between carrier injection and light emission is approximately microseconds or less. These characteristics are considered suitable for a flat panel display element.
Such a light emitting element is formed in a film shape. Thus, surface emission can be easily obtained by forming a large-area element. This characteristic is hard to be obtained by a point light source typified by an incandescent lamp or an LED or a line light source typified by a fluorescent lamp. Therefore, the above described light emitting element has high utility value also as a surface light source applicable to lighting or the like.
Thus, the current-excitation light emitting element using the light emitting organic compound is expected to be applied to a light emitting device, lighting, or the like. However, there are still many problems. As one example of the problems, reduction in power consumption is given. It is an important issue to reduce a drive voltage of the light emitting element in order to reduce power consumption. Since emission intensity of the current-excitation light emitting element depends on the amount of current flowing therethrough, it is necessary to conduct a large amount of current at low voltage in order to reduce a drive voltage.
It has been attempted so far to provide a buffer layer in contact with an electrode as a technique for reducing a drive voltage. Specifically, it is known that a drive voltage can be reduced by providing a buffer layer using an aromatic amine compound at an interface with an anode (for example, Reference 1: Y. Shirota et al., Applied Physics Letters, Vol. 65, 807-809 (1994)). The aromatic amine compound used in Reference 1 has a high location of HOMO level and an approximate value to a work function of an electrode material for forming the anode. Therefore, a hole injection barrier can be lowered. Accordingly, a large amount of current can flow at relatively low voltage.
Another method is also reported, in which a layer, conductivity of which is increased by adding electron-accepting molecules to a hole transporting high molecular weight material, is used at an interface with an anode (for example, Reference 2: A. Yamamori et al., Applied Physics Letters, Vol. 72, 2147-2149 (1998)). A drive voltage can also be reduced by using such a structure.
However, there is a problem in that such an organic compound which can lower a hole injection barrier as described in Reference 1 is limited, and heat resistance of the material is generally not high. The same applies to such an electron accepting molecule as described in Reference 2.
Conventionally, even if an organic compound which can lower a hole injection barrier is used, the hole injection barrier cannot be made to disappear substantially, and current-voltage characteristics of the light emitting element are controlled by injection (in other words, current-voltage characteristics in which a Schottky injection mechanism is dominant). Therefore, there is limitation on further reduction in a drive voltage.
Further, when a material which does not have a high work function is used for an anode, a hole injection barrier thereof is more increased. Therefore, there is another limitation in that a material having a high work function needs to be used as an electrode material for forming the anode in order to prevent an increase in drive voltage of the light emitting element. In other words, this leads to a problem in that general-purpose metal such as aluminum, which does not have a high work function, cannot be used for the anode.